Synthesis and Structure of Phosphanylated Bis-Triazoles as Potential Ligands for Chiral Catalysts

Article abstract of DOI:10.1007/s10593-014-1624-y

Potential atropisomeric ligands, diphenylphosphane derivatives of bis-triazoles, were synthesized by direct phosphanylation of the starting bis-triazoles with chlorodiphenylphosphane. Their crystal structure was elucidated by X-ray diffractional analysis.

Full text of DOI:10.1007/s10593-014-1624-y

DOI 10.1007/s10593-014-1624-y
Chemistry of Heterocyclic Compounds, Vol. 50, No. 11, February, 2015 (Russian Original Vol. 50, No. 11, November, 2014)
SYNTHESIS AND STRUCTURE OF PHOSPHANYLATED
BIS-TRIAZOLES AS POTENTIAL LIGANDS FOR CHIRAL
CATALYSTS
A. A. Kirilchuk1, A. A. Yurchenko1, Yu. G. Vlasenko1,
A. N. Kostyuk1, and A. B. Rozhenko1,2
*
Potential atropisomeric ligands, diphenylphosphane derivatives of bis-triazoles, were synthesized by
direct phosphanylation of the starting bis-triazoles with chlorodiphenylphosphane. Their crystal
structure was elucidated by X-ray diffractional analysis. The conformations of these compounds were
modeled by DFT calculations.
Keywords: BINAP, 1,2,4-triazole, phosphanes, atropisomerism, DFT, X-ray structural analysis.
Asymmetric catalysis is a new and rapidly developing discipline in the field of organic chemistry. A search
of new chiral catalysts has become the key step towards a production of enantiomerically pure drugs, pesticides,
etc. Atropisomeric compounds excel among such catalysts and their chirality arises due to the hindered rotation
about a single bond allowing separation of individual chiral conformers. Atropisomeric compounds can be used as
catalysts in organic synthesis both in pure form [1, 2] and as transition metal complexes.
Substituted biaryls are a classical example of structures with a chiral axis [3]. The first effective
atropisomeric catalyst based on 2,2'-bis(diphenylphosphano)-1,1'-binaphthyl (BINAP) was prepared in 1980 [4].
In the last the next years such catalysts have evolved from the laboratory to the industrial usage [5, 6].
Substituting naphthalene groups by heteroaromatic moieties in atropisomeric diphosphanes is interesting from
both for varying electronic properties and changing the geometrical and configurational stability of such ligands
[7]. Previously, a series of bis(heteroaryl)diphosphane were synthesized as ligands for the catalysts, which
demonstrated high activity and enantioselectivity in the model reactions [8, 9]. In these ligands, pyrole, indole,
thiophene, and furan rings were used as a the scaffolds. Similar bis(1,2,4-triazol)diphosphanes, also possessing a
chiral axis, have not been synthesized previously. Attractively, bis-triazole skeleton is synthetically accessible,
and, what is significant, its phosphanylation is highly selective.
The main aim of this study was the synthesis of the atropisomeric bis-triazolediphosphane ligands,
examination of their structure in the solid state, study their most favorable conformations by quantum-chemical
calculations, and evaluation of the energy barrier to racemization.
_______
*To whom correspondence should be addressed, e-mail: iamkaant@gmail.com, a_rozhenko@ukr.net.
1Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmans'ka St., Kyiv 02094,
Ukraine.
2Bielefeld University, 25 Universitätsstraße, Bielefeld D-33615, Germany.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1697-1705, November, 2014.
Original article submitted May 22, 2014; revision submitted August 22, 2014.
0009-3122/15/5011-1559©2015 Springer Science+Business Media New York
1559

Unsubstituted bis(1,2,4-triazole) 3 was synthesized according to the published procedure [10]. Its
2,2'-diphenyl derivative 6 was prepared by oxidative cleavage of pyridazine ring in the tricyclic compound 5
[11]. Conditions described in literature (KMnO4 in aqueous pyridine) did not allow achieving the expected
results. However, we found the reaction to proceed smoothly in aqueous KOH solution. Employing these
conditions, diphenylbistriazole 6 was synthesized in 69% yield.
NMe2
N
N
N
N
N
N
4 equiv. Ph 2PCl
Et 3N, Py
NMe2
PPh2
PPh2
PPh2
1
N
N
N
N
+
room temp., 3 days PPh2
N
N
N
N
N
N
3
4a
N
N
2
N
N
N
N
N
N
N
N
2 equiv. Ph 2PCl
KMnO 4
KOH, H 2O
Ph
Ph
Ph
Ph
PPh2
PPh2
Ph
Ph
Et 3N, Py
N
N
N
N
N
N
room temp., 1 day
60°C, 6 h
N
N
N
N
5
6
4b
Fig. 1. Molecular structure of compound 4a represented as thermal vibration ellipsoids with 50% probability
(hydrogen atoms not shown).
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Phosphanylation of bis-triazoles 3, 6 with chlorodiphenylphosphane in pyridine in the presence of
triethylamine during several days at ambient temperature or 6 h at 60°С according to published procedure [13]
afforded high yields of compounds 4a,b. The synthesized products 4a,b are colorless crystals, moisture
insensitive, and stable to hydrolysis by refluxing in moist methanol for 1 h. They are also highly stable in
regards to oxidizing agents. For instance, compound 4a did not add sulfur in refluxed benzene. On the other
hand, an interaction of compound 4a with hydrogen peroxide in benzene causes the cleavage of the C–P bond
with the formation of Ph2POH.
Phosphorus signals in 31P NMR spectra of phosphanes 4a,b (-42.6 and -39.4 ppm, respectively) appear
upfield in comparison with analogous signals in the known C-diphenylphosphanotriazoles (-30 ÷ -33 ppm) [13-15].
In the 13C NMR spectrum a doublet at 155.5-156.0 ppm (1JС–Р ≈ 16 Hz), corresponding to the C-3 carbon atom, is
characteristic.
Monocrystals of compounds 4a,b were grown from toluene and benzene, respectively; the structure of
these compounds was determined by X-ray structural analysis (Figs. 1, 2).
In the structure of compound 4a the lone electron pairs of the phosphorus atoms in all four phosphane
groups point to the neighbouring triazole ring. Atoms P(1), P(15), P(31), P(44) have a pyramidal configuration:
the sum of the corresponding valence angles (ΣP) is 298.6(3), 302.3(3), 298.6(3), and 300.4(4)°. The N(12)
atom possesses a trigonal-planar configuration (ΣN = 359.9(3)°). In contrast, the environment of the N(13)
atom is slightly out-of-plane (ΣN = 356.9(6)°). The planes of the two triazole rings are almost orthogonal –
the torsion angle between them comprises 83.16°.
The molecular structure of compound 4b possesses a C2 symmetry. The triazole rings are planar, the
deviations of atoms from the plane C(1)N(1)C(2)N(2)N(3) do not exceed 0.009 Å. The torsion angles between
the planes of the central five-membered rings and phenyl groups C(3–8), C(9–14), and C(15–20) are 39.76,
82.86, and 62.50°, respectively. The angle between the planes of the two triazole rings is 75.1°. The atom P(1)
possesses a pyramidal configuration, the sum of valence angles on the phosphorus atom ΣP = 306.9(6)°.
Fig. 2. Molecular structure of diphosphane 4b represented as thermal vibration ellipsoids with 50% probability
(hydrogen atoms not shown).
The conformation of a potential ligand in the gas phase and in solution can differ from its structure in
the solid state, which strongly influences its complexation ability. Quantum-chemical calculations for isolated
molecules of compounds 4a,b were performed using BP86 [16, 17] and B97-D [18] DFT functionals. The latter
1561

functional, which includes empirical corrections for electron dispersion effects, is able to model properly
π,π-interactions in the molecules of compounds 4a,b.
As indicated by calculations in the RI-BP86/TZVP approximation, the local minimum in energy for
compound 4a is the C2 symmetrical conformation. The lone electron pairs of all phosphorus atoms are turned to
the side of the N–N bond. Optimization of the geometry of the C2-symmetrical molecule using Grimme's B97-D
functional leads to a conformation where two pairs of phenyl groups are almost parallel to each other, which
implies a π,π-stacking interaction between them (Fig. 3a). Triazole rings are twisted to each other by 84.7°.
Optimization of the experimental structure without restrictions in symmetry leads to an asymmetric structure
(C1) (Fig. 3b), where one of the phosphane groups is turned around forming another π,π-stacking interaction
between two phenyl groups and triazole ring, as well as an additional СН,-bonding. The triazole rings in this
structure are turned by approximately 80° to each other. Noteworthy, the total energy of this structure is only by
1.3 kcal/mol lower than that calculated for above-mentioned С2-symmetrical structure. However, the difference
of free Gibbs energies (ΔG) calculated under standard conditions (T 298.15 K, p 0.1 MPа) makes the
С2-symmetrical structure slightly more favorable (ΔG -0.2 kcal/mol). Presumably, the large number of
substituents taking part in the interaction makes the С1-structure more rigid, which lowers its entropy, evening
the free energy values.
Fig. 3. Structures of compound 4a, optimized at the B97-D/TZVP level of approximation: a) С2-symmetrical,
b) asymmetrical (С1) (hydrogen atoms not shown, presumed ,-stacking interactions shown by arrows).
Fig. 4. Structures of compound 4b, a) optimized at the BP86/TZVP approximation and b) at the B97-D/TZVP
level of approximation (hydrogen atoms not shown, probable ,-stacking interactions marked by arrows).
The structures of compound 4b, optimized at the BP86/TZVP and B97-D/TZVP levels of approximations,
differ markedly as well. For the former case, optimization yields a С2-symmetrical structure (Fig. 4a).
Optimization within the RI-B97-D/TZVP approximation leads to an asymmetrical structure enabling two
1562

,-interactions between aromatic fragments: between the phenyl groups of the diphenylphosphane groups, and
between the phenyl rings connected to the triazole fragments (Fig. 4b). Benzene rings in these pairs are at
distances of 4.66 and 4.55 Å, slightly displaced to each other, and rotated by 27 and 37°, respectively. In the
structure calculated using the BP86 functional the two triazole rings are almost perpendicular (dihedral angle
about 87°), while method RI-B97-D provides a value for the dihedral angle of about 70°. Geometry
optimization for compound 4b taking into account dispersional interactions and with fixed С2-symmetry yields
the structure which does not correspond to the true local minimum in energy.
The phenomenon of atropisomerism arises from hindered rotation about the bond connecting two
aromatic fragments. In the case of compounds 4a,b the presence of sterically bulky groups accounts for a high
barrier for rotation about the N–N bonds. We located the transition state structures for this rotation and
determined their energy. The calculated activation energy values (ΔG≠ 40.5 and 42.5 kcal/mol) preclude
racemization under ordinary conditions and allow to predict that phosphanylated bis-triazoles could be used as
atropisomeric ligands.
For the characterization of the synthesized structures as potential ligands, we compared the spatial
arrangement geometry of the coordination centers, phosphane groups, with that of the above-mentioned BINAP
by the X-ray structural data [19]. Similar to structures 4a,b, the lone electron pairs of the phosphorus atoms in
BINAP molecule are also aligned almost parallel to the С–С bond between the naphthyl groups towards the
second naphthyl group. This peculiar geometry facilitates the efficient interaction of the transition metal atom
with both phosphorus atoms. The P···P distance in the BINAP molecule is 4.21 Å, whereas it is 4.10 Å in the
С2-symmetrical structure of compound 4a (in С1-symmetrical conformation these distances are about 4.1 and
4.6 Å), while in structure of compound 4b the corresponding values is 4.76 Å. The found Р···Р distances in the
studied structures in combination with their low deformation energies in symmetrical structures predicts for
them a potential ability to form complexes with transition metals. Besides, the nitrogen atoms of the triazole
rings can also take part in the complex formation with metal atoms.
Thus, we have for the first time synthesized bistriazolylphosphanes and studied their structure in the
solid state by X-ray structural analysis. Comparison of the distances between the coordination centers in the
obtained compounds with the distances in the classical diphosphane ligand BINAP shows their similarity, which
allows to predict the possibility of using these bis-triazoles as atropisomeric ligands in preparation of novel
chiral metal complex catalysts.
EXPERIMENTAL
1H NMR spectra were recorded on Bruker Avance DRX-500 (500 MHz, compound 4a) and Varian
VXR-300 (300 MHz, compound 4b) spectrometers. 13C NMR spectra were obtained on a Bruker Advance
DRX-500 (125 MHz) spectrometer. 31P NMR spectra were recorded on a Varian Gemini-200 (81 MHz)
1
spectrometer. All spectra were obtained in CDCl3 as the solvent; TMS was used as the internal standard for H
and 13C, while 85% H3PO4 was used as the standard for 31P. Melting points were determined on an
electrothermal melting point apparatus and are uncorrected. All operations with moisture- and air-sensitive
substances were performed under dry argon atmosphere in a flame-dried glass vessel. Solvents were purified
and dried according to the standard routines.
3,3',5,5'-Tetrakis(diphenylphosphano)-4,4'-bis(1,2,4-triazole) (4a). A solution of Ph2PCl (3.40 g,
15.4 mmol), 4,4'-bis(1,2,4-triazole) (3) (0.50 g, 3.7 mmol), and Et3N (2.22 ml, 1.60 g, 15.8 mmol) in pyridine
(5 ml) was stirred at room temperature for 3 days, the formed precipitate was filtered off and washed with
pyridine. The filtrate was concentrated under reduced pressure, and the residue recrystallized from PhMe. Yield
2.60 g (82%), yellowish crystals, mp >250°C. 1H NMR spectrum, δ, ppm: 7.19-7.32 (40H, m, H Ph). 13C NMR
spectrum, δ, ppm (J, Hz): 128.1; 129.2; 132.1; 133.8 (t, J = 11.3); 155.5 (m). 31P NMR spectrum, δ, ppm: -42.6.
1563

3,3'-Bis(diphenylphosphano)-5,5'-diphenyl-4,4'-bis(1,2,4-triazole) (4b). A solution of Ph2PCl
(0.43 g, 2.0 mmol), 3,3'-diphenyl-4,4'-bis(1,2,4-triazole) (6) (0.25 g, 0.9 mmol), and Et3N (0.28 ml, 0.20 g,
2.0 mmol) in pyridine (2 ml) was stirred at 60°C for 6 h, cooled to room temperature, and the precipitate filtered
off and washed with pyridine. The filtrate was concentrated, the residue was dissolved in CHCl3 and washed
with water. The organic phase was dried over Na2SO4 and concentrated. The residue was recrystallized from
1
PhH. Yield 0.44 g (78%), colorless crystals, mp 249-250°C. H NMR spectrum, δ, ppm: 7.05-7.40 (30H, m, H
13
Ph). C NMR spectrum, δ, ppm (J, Hz): 124.0; 127.1; 128.6 (m); 129.0; 129.6; 130.1; 130.9; 131.7; 134.1 (dt,
J = 123.0, J = 11.3); 153.2; 156.0 (d, J = 16.3). 31P NMR spectrum, δ, ppm: -39.4.
3,3'-Diphenyl-4,4'-bis(1,2,4-triazole) (6). KMnO4 (0.4 g, 2.5 mmol) was added to a suspension of
compound 5 (0.5 g, 1.6 mmol) in aqueous KOH (0.5 g in 7 ml Н2О). The mixture was stirred at room
temperature for 1 day. EtOH was added to remove the excess of KMnO4, precipitate of MnO2 was filtered and
washed with hot PhH. The aqueous filtrate was concentrated to dryness and extracted with hot PhH. The
combined benzene phases were dried over Na2SO4, and evaporated to dryness. Yield 0.32 g (69%), colorless
crystals, mp 207-208°C (PhH). The NMR spectra of the obtained compound 6 agrees with published data [11].
DFT Calculations. All structures were fully optimized without symmetry constraints (except for
specially mentioned cases) using the TURBOMOLE v. 6.2 software package [20, 21]. DFT RI-BP86 [16, 17]
and RI-B97-D [18] functionals were used for geometry optimization. The TZVP basis set used by default in
TURBOMOLE program is a modification of "triple-zeta" TZV basis functions [22] with added polarization
functions. Fine convergence criterion for SCF-calculations (SCFConv = 1.0×10-8 Hartree) in combination with
the highest integration accuracy (grid = 5) were used for geometry optimization. Vibrational frequencies
calculated by a numerical method were used to check if the optimized structures are real energy minima. The
PyMOL software [23] was used for graphical representation of the optimized structures.
TABLE 1. Principal Crystallographic Parameters for Compounds 4a,b
Parameter
Compound 4a
13.1935(4)
Compound 4b
12.1072(6)
Cell parameters
a, Å
b, Å
19.2616(5)
25.052(1)
c, Å
17.9868(5)
12.3979(7)
α, deg
90
90
β, deg
104.308(2)
104.823(3)
γ, deg
V, Å3
90
90
4429.2(2)
3635.2(3)
Z
4
4
Dcalc., g·cm–3
Crystal system
Space group
μ, cm–1
1.309
1.200
Monoclinic
Monoclinic
P21/c
P2/c
0.215
0.156
F(000)
1816
1368
Miller indices
16  h  -13, 24  k  -18,
17  l  -22
15  h  -14, 31  k  -31,
12  l  -15
θmax, deg
26.42
26.56
Reflections:
collected
32848
9149
5810
0.060
559
17765
3778
2422
0.044
217
independent
in refinement (I  3(I))
R(int)
Number of refined
parameters
R1(F)
0.0428
0.0800
0.0847
1.041
Rw(F)
0.0414
GOF
Δρmin / Δρmax, e·cm–3
1.1379
0.44/-0.34
0.99/-0.60
1564

Scanning of the potential energy surface of the rotational energy around N–N bond was carried out using the
GAUSSIAN-03 software [24] at the BP86/6-31G(d) approximation level. Locating the corresponding transition
states was performed according to the standard procedure in the BP86/6-311G(d,p) approximation (basis sets used
by default in the GAUSSIAN-03 software). To obtain the activation energy values corresponding to local energy
minimums, structures 4a,b were reoptimized in an analogous approximation. Vibrational frequencies for all
structures were calculated by an analytical method.
X-ray structural analysis of monocrystals of compounds 4a,b were performed at -100°С on a Bruker
Smart ApexII (λMoKα-beam) diffractometer. Both structures were solved by the direct method and refined by
the least-squares technique in the full-matrix anisotropic approximation using SHELXS97 and SHELXL97
software [25, 26]. Chebyshev weight function was used in the refinement. The principal crystallographic data
for compounds 4a,b are presented in the Table 1. In both structures all hydrogen atoms were identified from the
differential electron density synthesis and are included in the refinement with fixed positional and thermal
parameters.
The full set of X-ray structural data for compounds 4a,b was deposited in Cambridge Crystallographic
Data Center (deposits CCDC 1003773 and CCDC 1003774, respectively).
The Supplementary materials containing detailed calculations data for isolated molecules of compounds
4a,b, as well as Cartesian coordinates for all optimized structures are available for authorized users.
The authors are grateful to Prof. Uwe Manthe and Dr. Thorsten Tönsing of Bielefeld University,
Germany, for providing access to computer cluster and GAUSSIAN-03 and TURBOMOLE software, as well as
for technical support of calculations.
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